This invention relates to adjustable down-hole tools employed in the oil and gas drilling industry.
Drill string stabilisers, under reamers and fishing tools are some of the down hole tools that require activation when they are in a given position down hole to make them operative, and deactivation when they are to be withdrawn, or repositioned or indeed simply to go into a different operating condition.
Taking stabilisers as an example, these tools centralise drill strings with respect to the hole drilled. They normally comprise a sub assembly in the drill string. The stabiliser has a plurality of blades, (usually three and usually spirally arranged), whose edges are adapted to bear against the bore-hole. The blades are not complete around a circumference of the drill string so that the return route for drilling mud pumped down the bore of the drill string is not blocked. In order to control the direction of drill bits, it is sometimes required that the stabiliser has variable diameter. Pistons in the blades are extendable to give the stabiliser a maximum diameter, which ensures that the drill string is central in the bore-hole. The drill bit, assuming the stabiliser is close behind the drill bit, is thus kept straight. However, if the pistons are withdrawn, then gravity can deflect the drill string so that it alters the inclination of the hole.
EP-A-0251543 describes a stabiliser that is activated by weight on the stabiliser from the drill string above it. Weight, or absence thereof, switches the stabiliser between activated and de-activated positions. The weight acts on a mandrel slidable in the bore of the stabiliser, which mandrel has ramps against which wedge-surfaces on the bases of the pistons slide. A mechanical detent is overcome by a compressive force on the stabiliser greater than a threshold value, so that unless substantial changes in weight act on the stabiliser, switching does not occur. This means that some variation in weight is permissable without changing the activation of the stabiliser. However, it is known that excessive changes in weight can occur unintentionally, possibly resulting in accidental activation and deactivation of the stabiliser.
It has been suggested to employ a rise in mud pump pressure to move the mandrel in the stabiliser. Changes in pressure switch the mandrel between different positions. Such a system is described in EP-A-0190529, in which a differential piston cooperates with a flow restrictor so that, if the fluid pressure rises beyond a low threshold, the piston (or flow restrictor) moves to rapidly and substantially increase the pressure differential across the piston which then drives the mandrel to activate the stabiliser. As a subsidiary feature the mandrel rotates on each stroke because the pads have pins which follow a barrel cam defined around the mandrel, which barrel cam has different steepness ramps so that the pads are extended different amounts. Unintentional variations in fluid pressure might also cause premature activation or deactivation.
GB-A-2263923 discloses a stabiliser control arrangement in which the object is to not be dependent on either fluid pressure or weight on the bit to maintain a stabiliser setting. This is achieved by lifting the drill string to positively disengage the locking mechanism, and then fluid pressure is employed to determine the stabiliser piston position. At the appropriate pressure the drill string is lowered to engage a lock, whereupon subsequent changes in fluid pressure have no effect on stabiliser position.
GB-A-2251444 has essentially the same aims as GB-A-2263923, except that, here, check valves prevent operation or deactivation of the stabiliser pads unless the pressure of the pump fluid exceeds or falls below upper and lower threshold values.
EP-A-0661412 has an arrangement similar to EP-A-0190529. The position of a control piston determines the pressure drop across the mandrel which therefore controls the position of the mandrel. The control piston has a barrel cam in which a pin of the housing slides, so that the piston is constrained to follow a course determined by the track. A junction in the track is provided so that, at an intermediate pressure, if the pressure is reversed the pin does not return to its starting point but goes up a branch to a lesser (or greater) extent than its starting point. The stabiliser is activated between upper and lower pressures and that the pressure be taken from one level to an intermediate level whereupon the direction of pressure change is reversed.
GB-A-2314868 describes an arrangement in which the mandrel is hydraulically operated between operative and inoperative positions. A first shoulder on the body of the stabiliser in which the mandrel slides has a serrated face. A facing shoulder on the body has a clutch face which is also serrated. Between the two faces is a sleeve which is axially fixed but rotationally freely slidable on the mandrel. On the edge of the sleeve facing the serrated edge of the body is series of knobs to engage the serrations and rotate the sleeve through a small angle when the sleeve is axially pressed against the serrations. On its other edge, it has a series of fingers to engage the clutch face and either catch on ridges of the clutch face, which are provided with stops to prevent further rotation of the sleeve, or they miss the stops and hit a sloping serration of the lower shoulder causing further rotation of the sleeve until its fingers coincide with long slots in the shoulder whereupon the sleeve permits the mandrel to go to its operative position.
Consequently, as pressure is alternated and the mandrel moves back and forth, when it first moves down, for example, it may rest on the ridges of the clutch face and prevent the mandrel from going to its operative position. When the pressure is released and the mandrel rises the knobs on the sleeve hit the serrations and turn the sleeve through a small angle; enough so that on the next stroke of the mandrel the fingers on the sleeve do not stay on the ridges. Instead, the fingers slide down the serrations of the clutch face and drop into slots therein. This movement takes the mandrel into its operative position. Finally on the return stroke, when the knobs again contact the serrated face the sleeve again rotates, repeating the cycle.
A problem with this arrangement, and with EP-A-0661412 is that the pressure which activates the stabiliser must be greater, of course, than the return force provided by springs, for example, which springs must themselves be very substantial in order to guarantee deactivation and overcome any jamming tendency which could occur through external pressure on the pistons. Consequently, there is wear on the components which are rotating, or causing the rotation, since they are simultaneously subject to substantial axial loads. Moreover, in the case of GB-A-2314868, because the fingers are the same components which result in rotation of the sleeve, they cannot be as substantial as their loading, particularly in an extended position, would ideally want them to be. Thus they may break.
GB-A-2314868 also discloses application of the mechanism described therein in relation to under reamers.